Add Expression.Source.

It will be used for generating some error messages, see #37.
Fixes #49.
Merge pull request #51 from velour/source

Add Expression.Source.
Add Expression.Check.
Check Literal and Identifier nodes.
Add constSpecView.Check.
Add a placeholder for ConstSpec.Check.

Add comments about what the method needs to do, and add a comment on
constSpecView.Check() noting that it expects its ConstSpec to have
been previously checked.